Simultaneous droplet impingement dynamics and heat transfer on nano-structured surfaces

This study examines the hydrodynamics and temperature characteristics of distilled deionized water droplets impinging on smooth and nano-structured surfaces using high speed (HS) and infrared (IR) imaging at We = 23.6 and Re = 1593, both based on initial drop impingement parameters. Results for a smooth and nano-structured surface for a range of surface temperatures are compared. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. The near surface average droplet fluid temperatures are evaluated for conditions of evaporative cooling and boiling. Also included are surface temperature results using a gold layered IR opaque surface on silicon. Four stages of the impingement process are identified: impact, boiling, near constant surface diameter evaporation, and final dry-out. For the boiling conditions there is initial nucleation followed by severe boiling, then near constant diameter evaporation resulting in shrinking of the droplet height. When a critical contact angle is reached during evaporation the droplet rapidly retracts to a smaller diameter reducing the contact area with the surface. This continues as a sequence of retractions until final dry out. The basic trends are the same for all surfaces, but the nano-structured surface has a lower dissipated energy during impact and enhances the heat transfer for evaporative cooling with a 20% shorter time to achieve final dry out.     

Numerical analysis of secondary droplets characteristics due to drop impacting on 3D cylinders considering dynamic contact angle

This study is investigating three-dimensional numerical simulation of a Newtonian droplet impact and break on two square cylinders based on dynamic contact angle of droplet at the spatial interface between two solid–fluid phases. The droplet impact details and morphology studied in the present work could provide ideas for the spray wall impingement modeling in the simulation of many industrial applications, such as spray painting and liquid cooling of surfaces. The droplet impact is investigated on two square cylinders in 9 different modes with different droplet diameters and physical conditions such as different positions of droplet. The volume of fluid (VOF) method was used with open-source software. The results have been compared and validated quantitatively and qualitatively with the experimental results. Results represent droplet diameter into cylinder dimension and velocity profiles are affected on number of broken droplets, break times and droplet deformation. Also, mean velocities of droplet after impact on two square cylinders at first break time were 0, 0.025, 0.12, 0.47, 0.11, 0.08, 0.2, 0.012, 0.19 m/s for cases 1–9, respectively. Moreover, in case 7 that droplet diameter into cylinder dimension was 2, the maximum number of break-up into secondary droplets was 10 drops that occurred for 4 times.

     

Dynamic behaviors of droplet impact and spreading: A universal relationship study of dimensionless wetting diameter and droplet height

A notable universal relationship has been proposed in the literature for the evolution of dimensionless droplet height and wetting diameter during the initial spreading stage of droplet impingement. In this study, this universal relationship was investigated by employing three sets of measurements. Sequential images were recorded, and the whole droplet profile ensembles were plotted to facilitate this study. These sets of experiments were designed by changing impact velocity, surface hydrophobicity, or solution property. The experimental results illustrate that the importance of parameters causing the data variation is in the order of surface hydrophobicity > initial impact velocity > surfactant on wetting diameter, and surface hydrophobicity ≈ initial impact velocity > surfactant on droplet height. No universal relationship was observed for dimensionless droplet height and wetting diameter.     

Droplet impact dynamics for two liquids impinging on anisotropic superhydrophobic surfaces

Droplet impingement experiments were performed on grooved hydrophobic surfaces with cavity fractions of 0, 80, and 93?% using droplets of water and a 50?%/50?% water/glycerol mixture. The influence of liquid viscosity, cavity fraction, and spreading direction, relative to the surface grooves, is explored qualitatively and quantitatively. The maximum droplet spread diameter, velocity of the rebounding jet, and the time delay between droplet impact and jet emission were characterized for Weber numbers, We, based on droplet impact speed and diameter, up to 500. The unequal shear stresses and contact angles influence the maximum spread diameters in the two primary spread directions. At We?>?100, the ratio of the spread diameter along the direction of the grooves to the spread diameter perpendicular to the grooves increases above unity with increasing We. The maximum droplet spread diameter is compared to recent predictive models, and the data reveal differing behavior for the two fluids considered. The results also reveal the existence of very high relative jet velocities in the range 5????We????15 for water droplets, while such jets were not observed for the more viscous mixture. Further, in the range 115????We????265, the water/glycerol jet formation dynamics are radically different from the water behavior. Most evident is the existence of two-pronged jets, which arise from the anisotropy of the surface and the unequal shear stresses and contact angles that prevail on the surfaces. It is these influences that give rise to differences in the maximum spread diameters in the two primary spread directions. Similar two-pronged jet emission was observed for water over the very narrow range of We from 91 to 96. The issuing jet velocities were also observed to increase with increasing cavity fraction for both fluids and over the entire range of We explored. Lastly, the elapsed time between droplet impact and jet emission decreased with increasing cavity fraction.     

Energy balance of droplets impinging onto a wall heated above the Leidenfrost temperature

This work is an experimental study aiming at characterizing the heat transfers induced by the impingement of water droplets (diameter 80–180 μm) on a thin nickel plate heated by electromagnetic induction. The temperature of the rear face of the nickel sample is measured by means of an infrared camera and the heat removed from the wall due to the presence of the droplets is estimated using a semi-analytical inverse heat conduction model. In parallel, the temperature of the droplets is measured using the two-color Laser-Induced Fluorescence thermometry (2cLIF) which has been extended to imagery for the purpose of these experiments. The measurements of the variation in the droplet temperature occurring during an impact allow determining the sensible heat removed by the liquid. Measurements are performed at wall conditions well above the Leidenfrost temperature. Different values of the Weber numbers corresponding to the bouncing and splashing regimes are tested. Comparisons between the heat flux removed from the wall and the sensible heat gained by the liquid allows estimating the heat flux related to liquid evaporation. Results reveal that the respective level of the droplet sensible heat and the heat lost due to liquid vaporization can vary significantly with the droplet sizes and the Weber number.     

Investigation of secondary droplet characteristics produced by an isooctane drop chain impact onto a heated piston surface

The physics of the impingement process of small hydrocarbon droplets is mostly unknown due to the difficulties of obtaining experimental data. In this publication, the impingement of small isooctane droplets on a hot piston surface is studied. The emphasis was put on the image-based investigation of the process of successive impingements and its influence on secondary droplet formation. The diameter and velocity of the secondary droplets were found to depend on the wall temperature and impingement frequency. Thus, the temporal interaction has a significant influence on the secondary droplet characteristics.     

Modelling spray impingement using linear stability theories for droplet shattering

This paper compares several linear‐theory‐based models for droplet shattering employed for simulations of spray impingement on flat wall surface or a circular cylinder. Numerical simulations are conducted using a stochastic separated flow (SSF) technique that includes sub‐models for droplet dynamics and impact. Results for spray impingement over a flat wall indicate that the linear theory applicable for a single droplet impact over‐predicts the number of satellite (or secondary) droplets upon shattering when compared to experimental data. The causes for the observed discrepancies are discussed. Numerical simulation results for spray impingement over a circular cylinder in cross flow are obtained and discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

An experimental study of a water droplet impinging on a liquid surface

An experimental study is presented for water droplet impingement on a liquid surface. The impaction process was recorded using a high-speed digital camera at 1,000 frames/s. The initial droplet diameter was fixed at 3.1 mm ± 0.1 mm, and all experiments were performed in atmospheric air. The impact velocity was varied from 0.36 m/s to 2.2 m/s thus varying the impact Weber number from 5.5 to 206. The impacted liquid surface consisted of two fluids, namely water and methoxy-nonafluorobutane, C₄F₉OCH₃ (HFE7100). The depth of the water and HFE7100 pool was varied from 2 mm to 25 mm. The collision dynamics of water in the HFE7100 pool was observed to be drastically different from that observed for the water droplet impingement on a water pool. The critical impact Weber number for jet breakup was found to be independent of liquid depth. Water–HFE7100 impact resulted in no jet breakup over the range of velocities studied. Therefore, no critical impact Weber number can be defined for water–HFE7100 impact. Received: 27 June 2001/Accepted: 29 November 2001     

Numerical modeling and experimental measurements of water spray impact and transport over a cylinder

This study compares experimental measurements and numerical simulations of liquid droplets over heated (to a near surface temperature of 423 K) and unheated cylinders. The numerical model is based on an unsteady Reynolds-averaged Navier–Stokes (RANS) formulation using a stochastic separated flow (SSF) approach for the droplets that includes submodels for droplet dispersion, heat and mass transfer, and impact on a solid surface. The details of the droplet impact model are presented and the model is used to simulate water spray impingement on a cylinder. Computational results are compared with experimental measurements using phase Doppler interferometry (PDI). Overall, good agreement is observed between predictions and experimental measurements of droplet mean size and velocity downstream of the cylinder.     

A comparison between spray cooling and film flow cooling during the rewetting of a hot surface

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics to investigate the rewetting of a hot surface. The rewetting of the hot surface by spray cooling has been analyzed in previous works. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., drops having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, drop diameter and velocity, and impact point of the spray. The working feature of the spray is based on the varicose rupture of the liquid jet: imposing a periodic (symmetrical) perturbation with appropriate amplitude and frequency on the jet surface, the flow is “constrained” to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper, previous results with spray cooling are compared with experimental runs in which the spray injection is replaced with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and using a digital image system based on the video image registered during the runs.     

A numerical investigation of droplet impact on a heated wall in the film boiling regime

In this research, a novel approach is proposed to simulate the impingement of a water droplet on a superheated wall by solving conservation equations of mass, momentum and energy. Due to high temperature of the surface, a vapor layer is formed between droplet and surface. The vapor layer is captured using a very fine mesh near the surface. The level set method is applied for interface tracking and appropriate jump conditions are imposed at the interface by the ghost fluid method. The proposed algorithm is validated by comparing numerical results to the available experimental and analytical solutions. The effect of impact velocity, surfactant and polymer additives on the heat removal is studied. Simulations show that an increase in impact velocity enhances the heat removal. On the other hand, polymer additive has no significant effect while surfactant enhances dissipated heat by increasing the contact time or even by sticking the droplet on the surface.     

Modelling impingement of hollow metal droplets onto a flat surface

Despite many theoretical and experimental works dealing with the impact of dense melt droplets on the substrate during the process of thermal spray coating, the dynamics of the impingement of hollow melt droplet and the subsequent splat formation are not well addressed. In this paper a model study for the dynamic impingement of hollow droplet is presented. The hollow droplet is modelled such that it consists of a liquid shell enclosing a gas cavity. The impingement model considers the transient flow dynamics during impact, spreading and solidification of the droplet using the volume of fluid surface tracking method (VOF) coupled with a solidification model within a one-domain continuum formulation. The results for spreading, solidification and formation of splats clearly show that the impingement process of hollow droplet is distinctly different from the dense droplet. Study with different droplet void fractions and void distribution indicates that void fraction and void distribution have a significant influence on the flow dynamics during impact and on the final splat shape. The results are likely to provide insights for the less-explored behaviour of hollow melt droplets in thermal spray coating processes.     

Numerical simulations of droplet train and free surface jet impingement

The cooling behavior of the impingement of a droplet train, and free surface jets over a heated and pre-wetted surface is explored employing an Algebraic Volume-of-Fluid methodology. The code is based on a modified version of the two-phase numerical solver interFoam (OpenFOAM) (Trujillo and Lewis, 2012). Two versions of the free surface jet are studied. The first consists of a fully-developed profile exiting the nozzle, and the second is characterized by a uniform velocity distribution. Results show that both jet configurations have higher cooling performance than the droplet train locally and globally, with the fully-developed case being the most effective of the two jet arrangements. Locally, the performance is measured by radial profiles of the boundary-layer-displacement thickness and heat transfer coefficient. Globally, the cooling effectiveness is directly proportional to the surface area that resides within the high-convection region, i.e. before the boundary layer separation point. On a temporal basis, the liquid film within the impingement region of the droplet train exhibits pronounced variations in velocity magnitude and film thickness. This is directly attributed to the nature of continuous droplet impacts affecting the impingement region, and gives rise to an unsteady cooling and heating of the fluid near the wall. In contrast for the jets, the film and the corresponding free surface are nearly steady with only minor perturbations.     

Secondary atomization of water and isooctane drops impinging on tilted heated surfaces

The present paper reports an experimental study aimed at characterizing the effects of heat transfer on the secondary atomization, which occurs during droplet impact on hot surfaces at conditions reproducing those occurring at fuel injection in internal combustion engines. The experiments consider single isooctane and water droplets impacting at different angles on a stainless steel surface with known roughness and encompass a range of Weber numbers from 240 to 600 and heat transfer regimes from the film-vaporization up to the Leidenfrost regime. The mechanisms of secondary breakup are inferred from the temporal evolution of the morphology of the impact imaged with a CCD camera, together with instantaneous measurements of droplet size and velocity. The combination of a technique for image processing with a phase Doppler instrument allows evaluating extended size distributions from 5.5 μm up to a few millimetres and to cover the full range of secondary droplet sizes observed at all heat transfer regimes and impaction angles. Temporal evolution of the size and velocity distributions are then determined. The experiments are reported at impact conditions at which disintegration does not occur at ambient temperature. So, any alteration observed in droplet impact behavior is thermally induced. The analysis is relevant for port fuel injection systems, where droplets injected to impact on the back surface of the valves, behave differently depending on fuel properties, particularly when the use of alcohols is considered, even as an additive to gasoline.     

用格子Boltzmann方法模拟液滴撞击固壁动力学行为

首次用格子Boltzmann方法中的伪势模型对液滴撞击固壁的动力学行为进行了数值模拟.详细研究了液滴在壁面上的流动状态以及各种因素对撞击过程的影响.通过数值模拟得到:壁面的可润湿性越小,液滴越容易发生反弹,液滴的回缩速度越快;液滴的撞击速度越大,所得到的相对直径越大,回缩速度越快;液滴的粘性越小,所得到的相对直径越大;液滴的表面张力越大,液滴越容易发生反弹现象.另外,液滴的最大相对直径与We数满足一定的线性关系,这些结果与前人的理论预测和实验结果完全吻合.     

Lattice Boltzmann simulation of multiple droplets impingement and coalescence in an inkjet-printed line patterning process

Three-dimensional computations on the basis of the index-function lattice Boltzmann method are performed to simulate the process of multiple droplets impinging and coalescing into a line pattern on a solid substrate. The employed calculation model is validated by theoretical calculated values and experimental data from the literature. The influences of the equilibrium contact angle, droplet spacing and impinging velocity on the droplets impingement and coalescence behaviours are investigated. Numerical results demonstrate the width of the formed line depends significantly on the equilibrium contact angle and droplet spacing. The droplet spacing plays a significant role in controlling the coalescence moment of multiple droplets. The resolution of the printed pattern can be slightly increased with increase in impinging velocity.     

A two-dimensional splashing model for investigating impingement characteristics of supercooled large droplets

In this article, a two-dimensional (2D) splashing model is proposed to investigate the dynamics when supercooled large droplets (SLD) impinging on a wall surface in the aircraft-icing field. Energy conservation for droplet motion and impingement is used to capture the properties of the splashed droplets. A new statistical treatment of the droplet impinging energy and angle during the droplet-wall interaction is introduced in order to calculate the average dynamics of the SLD within a micro-control volume on wall surface. Based on the LEWICE predictions of droplet collection efficiencies and the available experimental ones, a new criterion for droplet splashing/deposition as well as a new formulation for the splashed mass is suggested. Lagrangian approach is adopted to describe the movement and impingement of SLD. The proposed model together with the previously developed droplet tracking method (DTM) for calculating droplet collection efficiency with the effect of droplet reimpingement constitute a relatively complete predicting approach of SLD impingement characteristics. Comparisons between the current predictions and the experimental observations, including SLD impingement over clean and contaminated airfoil surfaces as well as shapes of ice accretion in typical icing conditions, are carried out. Further, results obtained with the LEWICE splashing model are also plotted on the same graphs in order to assess the accuracy of the current splashing model in predicting SLD impingement. Results show that good agreement is achieved between the current predictions, including SLD impingement and ice accretion shapes, and the experimental ones. The predictions of the impingement distribution over contaminated surfaces obtained with the current splashing model show a much closer agreement with the experimental results than the ones obtained with LEWICE splashing model. For further investigation of SLD impingement, the properties of the droplet splashing and reimpingement during the ice accretion process are also addressed.     

液滴撞击超疏水壁面反弹及破碎行为研究

液滴撞击壁面时,壁面亲水性对液滴撞击壁面后的变化历程具有重要的影响。利用相界面追踪的复合Level Set-VOF方法对液滴撞击超疏水壁面的运动进行了研究。研究结果表明,撞击速度较小时,液滴撞壁后发生反弹;撞击速度较大时,液滴撞壁后会发生破碎现象;初始粒径的增大和表面张力的减小,有利于液滴撞壁后产生铺展破碎现象;撞击角度对撞壁后的液滴行为具有较大的影响。通过数值模拟,给出了一定条件下液滴垂直及倾斜撞击超疏水壁面反弹及破碎的临界条件。     

航空发动机轴承腔中油滴运动与沉积的特性分析

本文在获得轴承腔中气相介质流场的基础上,采用Lagrangian方法建立油滴在气相介质流场中运动的分析模型,通过瞬时步进法数值模拟油滴的运动过程,获得了油滴直径和旋转轴转速对油滴运动过程中的速度和轨迹影响的规律.基于获得的油滴与腔壁碰撞前的运动状态,以及结合油滴与腔壁的碰撞模型,实现了油滴直径和旋转轴转速对碰撞后油滴沉积率和动量转移率影响规律的分析.结果表明:油滴直径和旋转轴转速对油滴速度及轨迹,以及油滴沉积率及动量转移率都有很大影响,而且前者的影响更为明显.与国外同等条件下的试验结果对比表明,本文提出的油滴运动与沉积特性分析方法具有较好的可靠性和精度.碰撞前后油滴运动状态和沉积率及动量转移率的计算,为下一步油膜厚度和速度的计算,继而为轴承腔润滑设计和换热分析提供了初始条件.     

Numerical Investigation of the Time Scales of Single Droplet Burning

The effect of gas phase velocity fluctuations on single droplet burning is investigated numerically. The main objective of this study is to understand the effect of gas phase turbulence on nitric oxide formation in single droplet flames. Since the interaction of gas phase velocity fluctuations with droplet burning is of sequential character, a separate investigation of droplet momentum coupling and droplet burning is performed. Momentum coupling controls droplet relaxation against changes of the gas phase velocity along the droplet trajectory and, thereby, determines to what extend gas phase velocity fluctuations translate into droplet slip velocity fluctuations. This coupling effect acts as a high pass filter with a cutoff frequency determined by the droplet Reynolds number and diameter. In the simulation of single droplet burning detailed models for chemical reaction, diffusive species transport and evaporation are used. A significant effect of slip velocity fluctuations on the mean values of NO formation rate is observed. The effect of slip velocity fluctuations on the mean NO formation rate is frequency dependent. The frequency response of the droplet flame is similar to that of a low pass filter. The droplet flame time scale characterizing the response to slip velocity fluctuations is found to correlate with chemical time scales. This time scale is not affected by droplet diameter.